U.S. patent number 4,179,387 [Application Number 05/862,884] was granted by the patent office on 1979-12-18 for process for producing magnetic fe oxide.
This patent grant is currently assigned to Fuji Photo Film Co., Ltd.. Invention is credited to Tatsuji Kitamoto, Koji Sasazawa, Mahito Shimizu.
United States Patent |
4,179,387 |
Sasazawa , et al. |
December 18, 1979 |
Process for producing magnetic FE oxide
Abstract
A magnetic Fe oxide containing Co, and optionally, a divalent
metal, in which the oxidation degree indicated by the following
formula ranges from about 30% to 80%; ##EQU1## wherein y is the
amount of the Co, plus the divalent metal if present, in atomic %
contained in the magnetic Fe oxide, and R is the ratio of the
divalent Fe ion to the total Fe ions; and a process for producing a
magnetic Fe oxide containing Co, and optionally, a divalent metal,
and having an oxidizing degree of about 30 to 80%, which comprises
dehydrating and reducing goethite containing Co, and optionally,
the divalent metal, to obtain magnetic containing Co, and
optionally, a divalent metal, and oxidizing gradually the magnetite
at a relatively low temperature of lower than about 100.degree.
C.
Inventors: |
Sasazawa; Koji (Odawara,
JP), Shimizu; Mahito (Odawara, JP),
Kitamoto; Tatsuji (Odawara, JP) |
Assignee: |
Fuji Photo Film Co., Ltd.
(Minami-ashigara, JP)
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Family
ID: |
27286191 |
Appl.
No.: |
05/862,884 |
Filed: |
December 21, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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792956 |
May 2, 1977 |
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557431 |
Mar 11, 1975 |
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Foreign Application Priority Data
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Mar 12, 1974 [JP] |
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49-28423 |
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Current U.S.
Class: |
252/62.56;
252/62.57; 252/62.59; 252/62.6; 252/62.62; 252/62.63; 252/62.64;
G9B/5.257; G9B/5.269 |
Current CPC
Class: |
G11B
5/70689 (20130101); G11B 5/70626 (20130101) |
Current International
Class: |
G11B
5/706 (20060101); C04B 035/26 (); C01G 049/06 ();
C01G 049/08 () |
Field of
Search: |
;252/62.56,62.57,62.59,62.6,62.62,62.63,62.64 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Imaoka et al., "Ferrites:Proc. of the International Conf.," Jul.
1970, Japan, pp. 467-470..
|
Primary Examiner: Cooper; Jack
Attorney, Agent or Firm: Sughrue, Rothwell, Mion, Zinn and
Macpeak
Parent Case Text
The present application is a Divisional Application of U.S. Ser.
No. 792,956 filed May 2, 1977 now abandoned which was a
Continuation of U.S. Ser. No. 557,431 filed Mar. 11, 1975, now
abandoned.
Claims
What is claimed is:
1. A process for producing a magnetic Fe oxide having a coercive
force of at least about 300 Oe and and optionally an element
selected from the group consisting of Mg, Cu, Cr, Mn, Ni, Zn, Mo,
Sn, Sb, Te, Rh, Ba, La, Ce, W, Bi and mixtures thereof, the amount
of the element plus the Co being from about 0.5 to 33 atomic %, in
which the oxidation degree indicated by the following equation
ranges from about 30% to 80%: ##EQU3## wherein y is the amount of
Co, plus divalent metal if present, in atomic % contained in the
magnetic Fe oxide, and R is the ratio of the divalent Fe ions to
the total Fe ions,
which comprises
(i) dehydrating goethite containing Co and optionally said element
and (ii) reducing the dehydrated goethite containing Co, and
optionally said element at a temperature of about 300.degree. to
400.degree. C., to obtain magnetite containing Co, and optionally,
said element, followed by (iii) cooling the magnetite product of
step (ii) to room temperature under a flow of nitrogen gas, (iv)
covering the product of step (iii) with water, (v) removing excess
water from the wet magnetite so that the magnetite particles
produced in covering step (iv) have a moisture content of about 50
to 200% by weight relative to dried magnetite particles and (vi)
oxidizing the magnetite at a relatively low temperature of less
than 100.degree. C. to form a magnetic iron oxide having said
oxidation degree
2. The process of claim 1, wherein said reducing is with hydrogen
or town gas.
3. The process of claim 1, wherein said oxidizing is at a
temperature of about 40.degree. to less than 100.degree. C.
4. The process of claim 1, wherein said oxidizing is for a period
of about ten minutes to one week.
5. The process of claim 1, wherein said covering step (iv) is by
spraying the Co containing magnetite with water or by dipping the
Co containing magnetite into a water bath.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic Fe oxide powder suitable for
producing a magnetic recording member having stable magnetic
properties resistant to mechanical pressure and impact, and more
particularly to a magnetic Fe oxide powder suitable for producing a
magnetic recording member having less coercive force ageing,
especially a magnetic Fe oxide powder including Co or like divalent
metals (hereinafter designated as Co) and having a high magnetic
anisotropy and coercive force.
This invention also relates to a process for producing the magnetic
Fe oxide.
2. Description of the Prior Art
A Co containing magnetic Fe oxide powder is already known for
producing a magnetic recording member, which is obtained by adding
Co to a magnetic Fe oxide. The coercive force of the Co-containing
Fe oxide powder is high and the resulting recording member can be
used for high density recording. Methods of adding Co to Fe oxide
are disclosed, for example, in Japanese Patent Publication Nos.
15638/60, 10994/73, 6538/66 and U.S. Pat. No. 3,573,980.
When a Co containing .gamma.-Fe oxide powder or a Co containing
magnetite is formed into a magnetic tape or a like magnetic
recording body, the resulting magnetic recording member is
magnetically unstable to mechanical pressure and impact. That is to
say, the information and signals recorded on the magnetic recording
member are weakened by mechanical pressure and impact. Accordingly,
where the recorded tape is played repeatedly, the S/N ratio
gradually diminishes. Such a undesirable phenomenon is especially
marked in a recording member made from a Co containing magnetic Fe
oxide, whereas a recording member made from a usual Fe oxide or Cr
oxide does not exhibit this phenomenon as markedly. Such phenomenon
was already reported in (1) Akashi et al. Television, 18, p. 767
(1964), and (2) Kuroe et al. Magnetic Recording Research Conference
Text, No. MR 72-19 (Sept. 1972).
Studies have been made on improvements in the thermal and
mechanical stabilities of Co-containing magnetic Fe oxide. To
obtain thermal stability in Co-containing magnetic Fe oxide,
metallic ions other than Co ion are added to the oxide. Thus, a
Co-containing magnetic Fe oxide exhibiting a low reversible
coercive force change in the heated state can be produced (as
disclosed, for example, in Japanese Patent Publication No.
12175/68, and Japanese Patent Publication (O.P.I.) Nos. 27298/73,
27299/73). U.S. Pat. No. 3,573,980 discloses a method of producing
a Co-containing magnetic Fe oxide exhibiting less magnetization at
a high temperature by adding reduced Co to the Fe oxide.
The coercive force ageing and the magnetic stabilities to
mechanical pressure and impact have not yet been fully examined.
Improvement in these properties of a Co-containing magnetic Fe
oxide is an urgent problem to be solved.
SUMMARY OF THE INVENTION
Therefore, an object of this invention is to improve the magnetic
stability of a Co-containing magnetic Fe oxide to mechanical
pressure and impact and to provide a process for producing a stable
Co-containing magnetic Fe oxide.
Another object of this invention is to provide a magnetic Fe oxide
exhibiting a low coercive force ageing and a process for producing
the same.
It has now been discovered after much research that a Co-containing
magnetic Fe oxide becomes extremely stable to mechanical pressure
and impact at a suitable oxidation degree during a gradual
oxidizing treatment. The term "oxidation degree" is indicated by
the following relationship; ##EQU2## wherein y is the amount of the
Co, and a divalent metal if present, in atomic % contained in the
magnetic Fe oxide, and R is the ratio of the divalent Fe ions to
the total Fe ions.
This invention also provides a process for producing the magnetic
Fe oxide containing Co, and optionally, a divalent metal, and
having an oxidation degree of about 30 to 80% which comprises
dehydrating and reducing goethite containing Co, and optionally, a
divalent metal, to obtain magnetite containing Co, and optionally,
a divalent metal, and gradually oxidizing the magnetite at a
relatively low temperature of lower than about 100.degree. C.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE shows the relation between the oxidizing degree of the
magnetic Fe oxide and the magnetism reduction ratio.
DETAILED DESCRIPTION OF THE INVENTION
One feature of this invention is to oxidize gradually Co containing
magnetite at a relatively low temperature, e.g., about room
temperature (about 20.degree.-30.degree. C.) to 100.degree. C.
According to the process described in the U.S. Pat. No. 3,573,980,
the coercive force of the magnetic powder is increased by adding a
small amount of Co, e.g., 1 to 12% by weight to .gamma.-Fe.sub.2
O.sub.3 and reducing a part of the latter into FeO.
Another feature of this invention is to provide a process for
producing a magnetic Fe oxide containing a divalent metal and
exhibiting an oxidation degree of about 30 to 80% by dehydrating
and reducing goethite containing Co and, if desired a divalent
metal, to obtain magnetite containing Co and a divalent metal, if
present, and oxidizing gradually the latter at a relatively low
temperature range, i.e., below about 100.degree. C.
If the Co content in the Co-containing magnetic Fe oxide according
to this invention exceeds about 5 atomic %, the magnetic
characteristics are greatly reduced as compared with a conventional
Co-containing .gamma.-Fe.sub.2 O.sub.3 or magnetite. If the Co
content is not more than about 0.5 atomic %, the effect of Co to
increase the coercive force of the resulting magnetic Fe oxide is
scarcely observed.
The coercive force of the magnetic Fe oxide of this invention must
be higher than about 300 Oe, preferably higher than 1,500 Oe, and
such a coercive force can be controlled by changing the Co content.
For example, when 1.5 atomic % of Co is added to the Fe oxide at an
oxidation degree of 50%, a coercive force of 510 Oe can be
obtained; if 2.5 atomic % of Co is added, a coercive force of 640
Oe can be obtained; and if 4.0 atomic % of Co is added, the
coercive force becomes 780 Oe. A suitable range for the Co present
in the magnetic Fe oxide is about 0.5 to 10 atomic %.
The magnetic Fe oxide powder according to this invention can
contain small amounts of Mg, Cu, Cr, Mn, Ni, Zn, Mo, Sn, Sb, Te,
Rh, Ba, La, Ce, W and Bi in addition to Co. A suitable amount of
these additional elements plus the Co in the magnetic Fe oxide can
range from about 0.5 to 33 atomic %.
The resultant Co-containing magnetic Fe oxide powder can be
acicular or granular. If acicular, each particle preferably has a
longer diameter of about 0.1 .mu.m to 1 .mu.m. If granular, the
maximum size of each particle is restricted by the increase of the
noise when used as a magnetic recording material, and the minimum
size is restricted by the reduction in the transfer effect and the
increase of the washing and filtering time during powder
production.
Acicular Fe oxide can be produced by processes as described in U.S.
Pat. Nos. 3,117,933, and 3,720,618 and Japanese Patent Publication
No. 6538/66.
Granular Fe oxide can be produced by preparing first an aqueous
solution of Fe (II) and Co (II) in the desired amounts, adding
sodium hydroxide, potassium hydroxide or ammonium hydroxide to the
aqueous solution to keep the pH thereof above about 10 and
oxidizing the aqueous solution with air, oxygen, nitrate or like
oxidants keeping the temperature of the solution at about
20.degree. to 100.degree. C.
The Fe oxides thus obtained (Fe.sub.3 O.sub.4.nH.sub.2 O,
.alpha.-FeOOH or .alpha.-Fe.sub.2 O.sub.3 obtained by baking the
latter, Fe.sub.3 O.sub.4, .gamma.-Fe.sub.2 O.sub.3, each containing
the desired amount of Co) are reduced with hydrogen or town gas at
about 300.degree. to 400.degree. C., preferably 350.degree. C., to
convert the oxides into Fe.sub.3 O.sub.4, which is cooled room
temperature (e.g., about 20.degree.-30.degree. C.) under a flow of
N.sub.2 gas so that the Fe.sub.3 O.sub.4 is not contacted with air
and then covered with water. For example, the cooled Fe.sub.3
O.sub.4 is sprayed with water or dipped in a water bath. After
removing the excess water, the obtained Fe.sub.3 O.sub.4 particles
contain usually about 50 to 200% by weight of moisture with respect
to dried Fe.sub.3 O.sub.4 particles. An aqueous solution of nitric
acid, sodium nitrate, ammonium nitrate or a like oxidant, sodium
hydroxide, ammonium hydroxide, chloric acid and sulfuric acid can
be used instead of the water. In such cases, the Fe.sub.3 O.sub.4
particles must be fully washed with water to remove the soluble
materials therein.
After that, the wet "Co-containing magnetite" thus prepared is
treated in a constant temperature vessel kept at about 40.degree.
to 100.degree. C. and under normal atmospheric pressure. If the
temperature in the vessel is not higher than about 40.degree. C.,
the treatment requires one week or longer. On the other hand, if
the temperature exceeds about 100.degree. C., the oxidation is
undesirably accelerated generating heat, thus a non-uniform product
is obtained. The time required for accomplishing the treatment is
about 10 min. to 1 week.
The binder used in the process of this invention for binding the
Co-containing Fe oxide particles can be a conventional
thermoplastic resin, thermosetting resin, or a mixture thereof.
Useful thermoplastic resins have a softening point of lower than
about 150.degree. C., a mean molecular weight of about 10,000 to
200,000 and a degree of polymerization of about 200 to 2,000, and
include the following polymers; vinyl chloride-vinyl acetate
copolymers, vinyl chloride-vinylidene chloride copolymers, vinyl
chloride-acrylonitrile copolymers, acrylic ester-vinylidene
chloride copolymers, acrylic ester-styrene copolymers, methacrylic
ester-acrylonitrile copolymers, methacrylic ester-vinylidene
chloride copolymers, methacrylic ester-styrene copolymers, urethane
elastomers, polyvinyl fluoride resins,
vinylidenechloride-acrylonitrile copolymers,
butadiene-acrylonitrile copolymers, polyamide resins, polyvinyl
butyral, cellulose derivatives (cellulose acetate butyrate,
cellulose diacetate, cellulose triacetate, cellulose propionate,
nitrocellulose), styrene-butadiene copolymers, polyester resins,
chlorovinyl ether-acrylic ester copolymers, amino resins, various
synthetic rubber resins and mixtures thereof.
These thermo plastic binder resins are described in Japanese Patent
Publication Nos. 6877/62, 12528/64, 19282/64, 5349/65, 20907/65,
9463/66, 14059/66, 66985/66, 6428/67, 11621/67, 4623/68, 15206/68,
2889/69, 17947/69, 18232/69, 14020/70, 14500/70, 18573/72,
22068/72, 22069/72, 22070/72, and 27886/72, and U.S. Pat. Nos.
3,144,352, 3,419,420, 3,499,789 and 3,713,887.
Useful thermosetting resins have a molecular weight of less than
about 200,000 as a coating solution, but the molecular weight
becomes infinity due to the condensation and addition reactions
occurring on heating the coating solution. Such resins are
preferably not softened or melted before they thermally decompose.
Examples of thermosetting resins are phenol resins, epoxy resins,
polyurethane hardening resins, urea resins, melamine resins, alkyd
resins, silicone resins, acrylic reactive resins, epoxypolyamide
resins, nitrocellulose-melamine resins, a mixture of high molecular
weight polyester resin and an isocyanate prepolymer, a mixture of a
metha crylic copolymer and a diisocyanate prepolymer, a mixture of
a polyesterpolyol and a polyisocyanate, urea-formaldehyde resins, a
mixture of a low molecular weight glycol, a high molecular weight
diol and triphenylmethanetriisocyanate, polyamine resins and the
mixtures thereof.
These resins are described in Japanese Patent Publication Nos.
8103/64, 9779/65, 7192/66, 8106/66, 14275/66, 18179/67, 12081/68,
28023/69, 14501/70, 24902/70, 13103/71, 22065/72, 22066/72,
22067/72, 22072/72, 22073/72, 28045/72, 28048/72 and 28922/72, and
U.S. Pat. Nos. 3,144,353, 3,320,090, 3,437,510, 3,597,273,
3,781,210, and 3,781,211.
The binder can be used individually or as mixtures. Other
ingredients, such as dispensing agents, lubricants, abrasive agents
and anti-static agents, can be added to the binder. The weight
ratio of the ferromagnetic powder to the binder can suitably range
from about 100:10 to 100:200.
Useful dispensing agents are caprylic acid, capric acid, lauric
acid, myristic acid, palmitic acid, stearic acid, oleic acid,
elaidic acid, linoleic acid, linolenic acid, stearolic acid or like
fatty acids represented by the formula R.sub.1 COOH (where R.sub.1
is an alkyl or alkenyl group having 11 to 17 carbon atoms), alkali
metal (Li, Na, K, etc.) or alkaline earth metal (Mg, Cs, Ba, etc.)
salts of these fatty acids, and lecithin. Higher alcohols having
more than 12 carbon atoms and the sulfuric esters thereof can be
used.
Such a dispersing agent is employed in the binder in a weight ratio
of the dispersing agent to the binder of about 10:100 to 20:100.
Suitable dispersing agents are described in Japanese Patent Nos.
28369,64, 17945/69 and 15001/73, and U.S. Pat. Nos. 3,387,993 and
3,470,021.
Useful lubricants are silicone oils, graphite, molybdenium
disulfide, tungsten disulfide, fatty acid esters of monocarboxylic
fatty acids having 12 to 16 carbon atoms and monohydric alcohols
having 3 to 12 carbon atoms, and fatty acid esters of
monocarboxylic fatty acids having more than 17 carbon atoms and
monohydric alcohols in which the total number of carbon atoms in
the ester is 15 to 28. 0.2 to 2.0 weight parts of such a lubricant
are generally employed per 100 parts of the binder. These
lubricants are described in Japanese Patent Publication Nos.
23889/68, and 28043/72, Japanese Patent Application Nos. 28647/67
and 81543/68, and U.S. Pat. No. 3,423,233.
The useful abrasives are fused alumina, silicon carbide, chromium
oxide, corundum, synthetic corundum, diamond, synthetic diamond,
garnet and emery (main components are corundum and magnetite). A
suitable abrasive has a mean particle size of about 0.05 to 2
.mu.m, preferably 0.1 to 2 .mu.m. Generally about 7 to 20 weight
parts of the abrasive particles are employed per 100 weight parts
of the binder. These abrasives are described in Japanese Patent
Application No. 26749/73.
The useful anti-static agents are saponin or like natural
surfactants, alkyleneoxides, glycerin, glycidol or like nonionic
surfactants, higher alkylamines, quaternary ammonium salts,
pyridine or like heterocyclic compounds, phosphonium, sulfonium or
like cationic surfactants, carboxylic acids, sulfonic acids,
phosphoric acids, anionic surfactants containing sulfuric acid
ester groups, phosphoric acid ester groups or like acidic groups,
aminoacids, aminosulfonic acids, sulfuric or phosphoric esters of
aminoalcohols or like ampholytic surfactants.
Some of these surfactant compounds used as the antistatic agents
are described in U.S. Pat. Nos. 2,271,623, 2,240,472, 2,288,226,
2,676,122, 2,676,924, 2,676,975, 2,691,566, 2,727,860, 2,730,498,
2,742,379, 2,739,891, 3,068,101, 3,158,484, 3,201,253, 3,210,191,
3,294,540, 3,415,649, 3,441,413, 3,442,654, 3,475,174 and
3,545,974, German Patent Publication (OLS) 1,942,665, British
Patent Nos. 1,007,317 and 1,198,450, R. Oda et al. Synthesis of the
Surface Active Agents and Their Applications, Maki Shoten (1964),
A. W. Perry, Surface Active Agents, Interscience Publications Inc.
(1958), T. P. Sisley Encyclopedia of Surface Active Agents, Vol.
2.2, Chemical Publishing Co., (1964), and Surface Active Agent
Handbook 6th, Edition, Sangyo Tosho K. K., (Dec. 20, 1966).
These surface active agents can be used individually or as
mixtures. The surfactant is used not only for inhibiting the
formation of stactic electricity but also for improving the
dispersing, lubricating and coating properties and the magnetic
properties of the resulting recording member.
The formation of the magnetic recording layer can be carried out by
dispersing the Fe oxide mixture in the organic solvent and applying
the resulting composition on a support. A suitable coating
thickness of the magnetic layer on the support ranges from about
0.5 to 20 .mu.m, preferably 2 to 15 .mu.m.
The non-magnetic support can have a thickness of about 2.5 to 100
.mu.m, preferably 3 to 40 .mu.m, for a tape, and suitable supports
are polyethylene terephthalate, polyethylene naphthalate or like
polyesters, polypropyrene or like polyolefins, cellulose
triacetate, cellulose diacetate or the like cellulose derivatives,
polyvinyl chloride or like vinyl resins, polycarbonate or like
synthetic resins, aluminum, copper or other metals, glass or
ceramics, etc.
Useful organic solvents for kneading the Fe oxide particles and
coating the resulting composition are acetone, methyl ethyl ketone,
methyl isobutyl ketone, cyclohexanone or like ketones, methanol,
ethanol, propanol, butanol or like alcohols, methyl acetate, ethyl
acetate, butyl acetate, ethyl lactate, glycol acetate monoethyl
ether, or like esters, diethyl ether, glycol dimethyl ether, glycol
monoethyl ether, dioxane or like ethers, benzene, toluene, xylene,
or like aromatic hydrocarbons methylene chloride, ethylene
chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin,
dichlorobenzene or like chlorinated hydrocarbons, etc.
For coating the Fe oxide containing mixture on a support, an air
doctor coating method, a blade coating method, an air knife coating
method, a squeeze coating method, a dip coating method, a reverse
roll coating method, a transfer roll coating method, a kiss coating
method, a cast coating method, and a spray coating method can be
used. The details of these coating methods are described in Coating
Engineering, pp 253-277, Asakura Shoten (Mar. 20, 1971).
The magnetic Fe oxide used in the FIGURE is a Co-containing Fe
oxide which contains about 4 atomic % Co and is mildly oxidized at
60.degree. C. The "magnetism reduction ratio" means the ratio of
the magnetism reduction amount in a magnetic tape made from a
magnetic Fe oxide with a 100% oxidation degree to that in a
magnetic tape made from a magnetic Fe oxide of any oxidation
degree. The magnetism reduction amount is indicated by the ratio of
the difference between the regenerated output (A) after running the
tape for a predetermined time period compressed with a roller with
a constant controlled pressure and that (B) immediately after
recording the latter (B). (A-B/B).
From the test results a Fe oxide containing more than 0.5 atomic %
Co exhibits a similar behavior to that of the Fe oxide shown in the
FIGURE. That is to say the magnetism reduction amount of the Fe
oxide is decreased in the range of an oxidation degree of about 30
to 80%.
The Co-contaning magnetic Fe oxide according to this invention
exhibits an extremely low magnetism and is capable of recording and
retaining stably information and signals as compared with a
conventional Co-containing Fe oxide and magnetite.
Now, this invention will be described with reference to the
following examples. Unless otherwise indicated, all parts,
percents, ratios and the like are by weight.
EXAMPLE 1
Ferrous sulfate (200 g) was dissolved in water (600 ml), and the
resultant solution was added to a solution prepared by disolving
sodium hydroxide (28.8 g) in water (1800 ml). Air was bubbled into
the solution mixture at room temperature to accelerate the reaction
therein until the solution was colored yellow, and then the
precipitate was filtered and again dispersed in water (2 liters).
The crystal nuclei had a diameter of 0.05 .mu.m and a length of 0.5
.mu.m, and the dispersed solution contained goethite (24 g). After
keeping the solution temperature at 50.degree. C., 2 liters of a
0.5 mol% ferrous sulfide aqueous solution containing Co sulfide
(5.2 mol%) was added to the dispersed solution at a constant rate
over about 8 hours while controlling the pH of the resulting
solution at 6.0 by adding a 2 N sodium hydroxide aqueous solution
thereto. Thus, each of the crystal nuclei grew to have a diameter
of 0.2 .mu.m and a length of 1.1 .mu.m.
The Co content of the goethite was found to be 4.0% by fluorescent
X-ray analysis. The obtained Co containing goethite was converted
into Co-containing .alpha.-Fe.sub.2 O.sub.3 by exposing the
goethite to an air flow at 300.degree. C. The Co-containing
.alpha.-Fe.sub.2 O.sub.3 was reduced in a hydrogen gas flow at
350.degree. C., cooled to room temperature, and removed exposed to
air, while mildly oxidizing the surface layer with a low oxygen
partial pressure to not cause a violent oxidation. The oxidation
degree was about 5%.
The obtained Co containing magnetite was placed in a drier kept at
60.degree. C. to oxidize under mild conditions for about 40 hours.
Thus, a magnetic Fe oxide having a coercive force of 720 Oe and a
oxidation degree of 25% was obtained. (Magnetic Material A)
EXAMPLE 2
A Co containing goethite obtained in the same manner as in Example
1 was exposed to an air flow at 300.degree. C. to convert it into a
Co-contaning .alpha.-Fe.sub.2 O.sub.3, which was then reduced in a
hydrogen gas flow at 350.degree. C. and then cooled. The obtained
Fe.sub.3 O.sub.4 was dipped in water without exposure to air, dried
in a drier at 60.degree. C. for 40 hours, and oxidized under mild
conditions. Thus, a magnetic Fe oxide having the coercive force of
780 Oe and an oxidation degree of 45% was obtained (Magnetic
Material B).
EXAMPLE 3
A Co containing magnetite (60 g), which was produced in the same
manner as in Example 1, was dispersed in 300 ml water, and ammonium
nitrate (30 g) was added to the dispersion. While a sodium
hydroxide aqueous solution was added to the resulting solution to
keep the pH at 9, the solution was agitated for 1 hour and then
washed with water. After drying for 24 hours at 60.degree. C., a Co
containing magnetic Fe oxide having a coercive force of 750 Oe and
an oxidation degree of 35% was obtained (Magnetic Material C).
EXAMPLE 4
A Co containing magnetite obtained in the same manner as in Example
1 was dipped in chloric acid (0.1 N aqueous solution) for 30 min.,
and then washed with water. The resulting magnetite was dried and
oxidized under mild conditions in a drier for 40 hours at
60.degree. C. The obtained Co containing magnetic Fe oxide
(Magnetic Material D) had a coercive force of 780 Oe and an
oxidation degree of 55%.
EXAMPLE 5
A Co containing magnetite obtained in the same manner as in Example
1 was dried and oxidized gradually at 60.degree. C. for 140 hours
in a drier. Thus, a Co containing magnetic Fe oxide produced
(Magnetic Material E) had a coercive force of 640 Oe and an
oxidation degree of 85%.
COMPARATIVE EXAMPLE 1
A magnetite which contained Co (4%) was cooled and dipped in a
solvent without exposure to atmospheric air (Magnetic Material F).
The oxidation degree was 0%.
COMPARATIVE EXAMPLE 2
A magnetite was oxidized in atmospheric air at 250.degree. C. and
.gamma.-Fe.sub.2 O.sub.3 containing Co (4%) was obtained (Magnetic
Material G). The oxidation degree was 100%.
EXAMPLE 6
Each of the Co containing magnetic Fe oxide Magnetic Materials A to
G (300 g) was mixed with the following composition
______________________________________ parts by weight
______________________________________ Vinyl Chloride-Vinyl Acetate
40 (87:13 by weight) Copolymer Epoxy Resin* 30 Silicone Oil** 5
Toluene Sulfonethylamide 7 Ethyl Acetate 250 Methyl Ethyl Ketone
250 ______________________________________ *epichlorohydrin and
diphenyl propane condensate having a hydroxyl group content of
0.29, a molecular weight of about 470; an epoxy content of 0.40; an
epoxy equivalent of 250 **dimethyl polysiloxane having a molecular
weight of about 1,000, a specific gravity of (25.degree. C.) of
0.94 and a viscosity (25.degree. C.) of 98 Centistokes
The obtained composition was well kneaded in a ball mill and
"Desmour L-75" (trade name, produced by Bayer A. G.; 75 wt.% ethyl
acetate solution containing an adduct of 3 moles of
toluenediisocyanate and 1 mole of trimethylol propane) (20 parts)
was dispersed uniformly in the composition to obtain a magnetic
coating composition. Each of the magnetic coating compositions was
applied on an ethylene terephthalate support of a thickness of 25
.mu.m so as to form thereon a dried magnetic layer of a thickness
of 10 .mu.m, and produce Tape Samples A to G corresponding to
Magnetic Materials A to G. Each of the obtained tapes was placed in
a 1,000 Oe magnetic field and dried to produce a magnetic tape.
Each of the resultant magnetic tapes were examined using a
magnetism reducing tester to measure the reduced magnetic amount
after running 300 times. The test results obtained are shown in the
following Table.
______________________________________ Reduced Oxidation Coercive
Saturation Magnetic Tape Degree Force* Squareness Magnetism* Amount
Sample (%) (Oe) Ratio* (Gauss) (%)
______________________________________ A 25 720 0.78 1360 17 B 45
780 0.79 1400 10 C 35 750 0.75 1350 13 D 55 780 0.82 1280 7 E 85
640 0.80 1310 18 F 0 540 0.75 1420 30 G 100 520 0.81 1230 35
______________________________________ *Measured in an external
magnetic field of 2,000 Oe.
Tape Samples A to E are typical examples of tapes produced using
mildly oxidized magnetic Fe oxide. Any degree of the oxidation can
be obtained by controlling the oxidizing temperature, time and
atmosphere. The relation between the magnetism reduction ratio and
the oxidation degree as shown in the FIGURE can be obtained using
any oxidizing process.
It will be apparent from the results in the above Table that the
Co-containing magnetic Fe oxide according to this invention is
magnetically stable to mechanical pressure and impact and suitable
for producing ageing resistant magnetic recording members.
The Co-containing magnetic Fe oxide is also useful for magnetic
recording tapes for image signals, voice signals and digital
information, magnetic recording sheets and magnetic recording
discs.
While the invention has been described in detail and with reference
to specific embodiments thereof, it will be apparent to one skilled
in the art that various changes and modifications can be made
therein without departing from the spirit and scope thereof.
* * * * *